The invention relates to a method and an apparatus for topping up cartridges, which must be filled to their edge with a liquid and be closed by a sealing membrane leaving a minimum of air between the surface of the liquid and the sealing membrane.
A cartridge of this kind is a cylinder ampoule of the kind comprising a cylindrical tube which has a first and a second end, the first end being end closed by a piston and at the second end having a neck part terminated by a circumferential flange against which a rubber membrane is pressed sealingly by a cap having means gripping behind the flange. Such ampoules are commonly filled with a liquid medicine preparation and are used in pen shaped injection devices by which set doses of the preparation may be injected until the ampoule is empty.
The filling of the ampoules is performed in a sterile zone in which a number of operating stations are disposed. To perform the filling quickly and precise the filling is often made in three steps. First about 40% of the content is by a maximal speed filled into the ampoule, thereafter the next 40% is added more slowly to prevent formation of foam, and finally the ampoule is topped up relatively slowly with the last 20% of the liquid.
Especially the topping up step must be carried out carefully to ensure that the ampoule is totally filled before it is closed with the sealing membrane. This may be obtained by filling until an overflow is detected by using a filling head which presses a gasket against the upper edge of the ampoule, the gasket having two openings, a feeding opening through which the liquid is fed to the ampoule and an overflow opening through which excessive liquid leaves the cartridge when the cartridge is full. When overflow is detected the filling is stopped. Alternatively liquid is sprayed into the cartridge through a filling needle placed a short distance above the opening of the cartridge. A suction needle ends immediately over the opening of the ampoule and sucks away exceeding liquid when the cartridge is full and the liquid begins rise as a drop on the upper end of the cartridge. A liquid level monitor is established by placing a light source at the a flange established at the upper opening of the cartridge, against which flange a closing membrane can be sealed, when the cartridge has been filled. The light from the light source passes trough the flange and the space surrounded by the flange and into a sensor. When said space is filled with liquid the transmission parameters for the light beam are changed and the sensor senses this change and sends a signal, which stops the filling of the cartridge. During the time from the sensor detects the change in the transmitted light until the filling is actually stopped the rest of the ampoule is filled and sufficient extra liquid is delivered to rise the level sufficiently to ensure that the ampoule is totally filled which is indicated by liquid being sucked away though the suction needle.
It is an objective of the invention to provide a better topping up method by which waste of excessive liquid is reduced or eleminated and the formation of air bubbles is minimised.
A method according to the invention is characterised by the steps
a) lowering a filling needle into the cartridge,
b) feeding liquid through the filling needle into the cartridge,
c) detecting when the cartridge is filled to its edge,
d) stopping the liquid flow when the cartridge is detected as being full,
e) lifting the filling needle out of the cartridge.
When the filling needle is lowered into the cartridge its tip is during the topping up placed in a smaller distance from the liquid surface in the cartridge or it even dips into the liquid from the initial filling process comprising one or two prefilling steps which are then succeeded by a topping up step.
The detecting at the edge may be obtained by passing a light beam immediately over the upper end of the cartridge and into a sensor. This way disturbance of the light beam due to irregularities in the glass flange is avoided. However, the liquid surface will inevitably rise over the edge of the cartridge as the stop signal to the pump is not sent until the detector detects such a rise. The liquid will rise as a drop only held by surface tension, but when the filling needle is lifted out of the cartridge the space which has been occupied by the needle will adopt the excessive amount of liquid forming the drop and the liquid level will fall to flush with the edge of the cartridge. This can be ensured by adjusting the distance the needle is lowered into the cartridge and the delay between the signal stopping the pump and the actual stopping of the filling.
When the filing needle is dips right into the liquid in the cartridge it is avoided that the jet of liquid from the filling needle entrains air into the liquid in the cartridge.
The invention further relates to an apparatus for performing the described method. Such an apparatus is characterised in that it comprises:
a filling needle which can be lowered to project into a cartridge,
a controllable liquid feeding device,
a detector detecting when the liquid level reaches the upper edge of the cartridge
to control the feeding device to stop feeding liquid to the cartridge when a set upper level is reached, and
a means for lifting the filling needle out of the cartridge when the filling is done.